A cleaning solution is used to remove limescale, CaCO 3, from bathroom surfaces. When the solution is sprayed onto the limescale, effervescence (fizzing) occurs and the solid limescale begins to disappear. This chemical reaction is needed because the limescale cannot be removed with water alone.
Which of the following statements about this chemical reaction are correct?
1 The cleaning solution is acidic.
2 The effervescence (fizzing) is caused by the release of hydrogen gas.
3 The pH of the reacting solution will go down as the reaction proceeds.
4 The salt produced in the reaction is more soluble than CaCO 3.

Answer:

2) C

CaCO3 is a salt of a strong base and weak acid, which is why it is basic in nature. In order to remove it, we need to use an acidic solution. So, statement 1 is correct.

The effervescence is caused by the release of carbon dioxide gas (which forms during the reaction) and not hydrogen gas. Statement 2 is wrong.

As the reaction proceeds, the pH does not go down as we have a base and an acid reacting. So statement 3 is wrong.

The reason we are using a cleaning agent is to remove the limescale. In order to do this, we need to convert the limescale into something more soluble. We know that a salt is formed when reacting a base and an acid. Therefore, the salt has to be more soluble than limescale so it can be removed from the bathroom surfaces. So, 4 is correct.